The present invention pertains generally to the display of video images and more particularly to a method for generating a plurality of moving objects on a video display screen in the context of a video amusement game.
One example of a system for causing images of objects to move about on the screen of a video display tube under the control of an operator in U.S. Pat. No. 3,793,483 issued Feb. 19, 1974, in the name of Nolan Bushnell entitled "Video Image Control System for Amusement Device" and assigned to the present assignee. This patent shows a separate motion counting or comparator circuit for each moving object to be displayed. In other words, with multiple objects multiple counters or comparators are required which can result in relatively complex and expensive circuitry.
A copending application, now U.S. Pat. No. 4,016,362, Ser. No. 626,665 filed Oct. 29, 1975 in the name of Stephen Bristow entitled "Multiple Image Positioning Control System and Method" and assigned to the present assignee provides a multiple object display system utilizing logic for ordering the various displayed objects.
The basic objective in the display of multiple objects is that the logic system must be able to control the intensity of one or more electron beams as a television monitor is scanned. The scanning rate for standard monitors is too fast for on-line output by low cost computers of all of the information needed to represent the image. Thus a buffer is required. The motion control logic discussed above is one type of buffer.
In one of the common existing systems, the logic or computer output is stored in a display memory of one or more television fields in length. The organization of the list is such that it is searched in time sequence with the television monitor raster. Thus, data required for the beam control is available at the time required. The limitation in this approach is the size of the memory. For a video display with 256 vertical and 512 horizontal possible positions, a memory of the product or 131,072 bits is required. If multiple objects are to be displayed and if each object must retain an identification code for later use, the memory must be increased by a multiple of the identification code length. Such a large memory is not economically feasible in games.
Another approach used in the graphics display field is the use of a pair of memories, for example, of the shift register type for the horizontal lines to be displayed. In operation while one horizontal television line is being displayed the command list is checked to see if any points are to be displayed during the next line. Actual video data is set in the first line memory while the second line memory is being displayed. In order to properly load the memory, hardware is required to calculate the "a" intersect of a display command. This approach requires extensive and costly control and memory circuits. It also does not retain the unique identification of the video for each object.